C++ Broker use of "--port 0" option

I've just opened QPID-4931[1] and a code review [2] to change the
behaviour of the --port 0 command line option to the C++ broker.

Essentially if you use this option after the change the broker will only
listen to a single address and you will need to use --interface to
select the address you require.

This option is only really useful for testing purposes and so I don't
expect this to have very much impact, but please do speak up if you use
this option for some purpose that I can't imagine and will be affected
by this change.

Note that in the future I'd like to entirely get rid of this option and
replace its uses with the --socket-fd option and the work that Alan has
done [3].

I'd like to get this in for 0.24 so speak up soon.

Thanks

Andrew

[1] https://issues.apache.org/jira/browse/QPID-4931
[2] https://reviews.apache.org/r/11915/
[3] https://reviews.apache.org/r/10872/